This invention relates to a photocurable composition and, more particularly, to a photocurable composition which is especially suitable for forming printing plates.
As a photocurable composition for forming printing plates in the conventional art, there has been, for example, proposed and disclosed in the Japanese Pat. No. 265544 a photocurable composition comprising a high molecular material, a polymerizable ethylenically unsaturated compound and a photosensitizer which is applied or molded onto a substrate to obtain a solid state relief image forming layer at room temperature. Said relief image forming layer is cured by the application of actinic rays and then the un-cured portion is washed away with aqueous alkali solution to obtain the desired printing plate.
Although said high molecular materials such as cellulose derivatives having oxyacid groups, acrylic polymer having oxyacid group and acidic polyvinyl alcohol have two characteristics that make it possible to keep the photocurable composition in the solid state at room temperature and that can allow the un-cured photocurable composition to wash away by using an aqueous alkali solution, they have poor compatibility with the polymerizable ethylenically unsaturated compound and reduce the photopolymerization velocity at the time of forming printing plate. Further, the printing plate obtained is inferior in respect of several properties such as toughness, flexibility, stiffness, abrasion resistance and water resistance.
In the meantime, as the polymerizable ethylenically unsaturated compound, there is, for example, an unsaturated resin, so to speak "acryl-urethane resin" which is disclosed in Japanese Patent Publication No. 43657/1973. Said acryl-urethane resin itself is excellent in mechanical and chemical properties such as abrasion resistance, adhesion, solvent resistance and curing property which is not effected very much by atmospheric oxygen.
Accordingly, in the conventional art, it was possible by mere incorporating the high molecular material into the acrylurethane resin which is in the liquid state at room temperature to obtain the photocurable composition having the foregoing two characteristics, however, the resulting photocurable composition was unable to make use of the above-mentioned features of the acryl-urethane resin thoroughly, so that the photocurable composition was also not always satisfactory in several properties such as curing property, abrasion resistance, adhesion and solvent resistance.